With the vehicle braking systems currently in production, there is no way to detect while using the vehicle, if not by way of external systems, the forces that are exchanged between the brake pads (or brake shoes for vehicles equipped with drum brakes) and the disc or drum element attached to the wheel that is being braked. This makes it impossible to detect and/or predict the occurrence of many drawbacks which can vary from abnormal brake pad wear due for example to the pads “touching” the disc even when braking is not taking place due to poor caliper adjustment, rather than the noise, vibration and undesirable squealing during braking. These problems can lead to malfunctions or even to premature and often unnecessary brake pad replacement.
KR2004-48957A attempts to solve the problem of the generation of noise and vibration during braking by arranging piezoelectric elements behind the metal support for the brake pad which is known as the “backplate”. These piezoelectric elements, as a result of the vibrations to which the backplate is subjected during use, generate electrical energy which is subsequently dissipated by supplying said energy to LEDs. By absorbing energy in this manner these vibrations are dampened.
More generally the use of piezoelectric elements (sensors or actuators) for experimental purposes is known in the art. For example arranging piezoelectric elements against the backplate and facing towards the outer sides of the brake caliper in order to detect the deformations of said backplate during braking bench tests or for counteracting the vibrations that are generated during braking by electrically supplying piezoelectric actuators such that said actuators then apply forces to the backplate that should counterbalance and thus dampen the vibrations.
For example, the SAE Technical Paper 2004-01-2773 describes a study conducted by the University of Darmstadt in which piezoelectric elements are coupled together with acceleration sensors and used both as sensors and actuators for the squealing suppression during braking. The same study points out however that the system described should be considered as a design aid tool and not as a system suitable for use in the mass production of brake pads.
EP1531110 and GB2478423 describe vehicle braking systems wherein piezoelectric sensors are respectively arranged on the brake disc or between the brake caliper piston and backplate so as to produce respectively either a signal that is used by an electric motor to adjust the position of the brake caliper piston or else to rapidly detect any signs of wheel lock during braking.
It should be noted in fact that at the moment it is considered to be extremely difficult, if not impossible, to have piezoelectric sensors in the vicinity of brake pads or on the brake pads themselves, due to the high temperatures and pressures to which brake pads are subjected during the production process or that are developed during braking, resulting in temperatures and pressures that are incompatible with most known piezoelectric sensors.
EP1431606B1 therefore describes a method for the measurement of forces applied to a friction layer where a functional layer, whose electrical resistance varies as a function of the forces applied to it, is associated with said friction layer, then the variation in functional layer electrical resistance is measured thus giving an indication of the amount of applied force.
EP1923592B1 instead describes a braking or friction element having a friction layer and a support plate with at least one capacitive sensor arranged between the friction layer and the support plate, the capacitance of which varies as a function of the force applied to said friction layer.
Finally US2006/0254868 describes a system similar to that of EP 1431606B1 wherein the variation in electrical resistance of a braking element friction layer, such as that of a brake pad, is measured directly.
These systems, which are based on the variation in capacitance or electrical resistance of a sensor or an entire functional layer are however very difficult to be made feasible in practice, because, on one hand they allow for relatively precise measurement of forces applied in static manner or in a very slow acting manner, but they are unable to detect rapid changes in force such as those that occur during braking; and, on the other hand, they also need to be continuously electrically supplied thus leading to associated energy consumption and considerably complicated construction in that electrical energy has to be supplied to relatively fast moving parts.